1. Field of Invention
This invention relates to communications systems. Specifically, the present invention relates to systems and methods for facilitating handoff in telecommunications systems.
2. Description of the Related Art
Cellular telecommunications systems are characterized by a plurality of mobile units (e.g. cellular telephones) in communication with one or more base stations. Signals transmitted by the mobile units are received by a base station and often relayed to a mobile switching center (MSC). The MSC in turn routes the signal to a public switched telephone network (PSTN) or to another mobile station. Similarly, a signal may be transmitted from the public switched telephone network to a mobile unit via a base station and a mobile switching center.
Each base station covers a `cell` within which a mobile unit may communicate. A cell covers a limited geographic area and routes calls from mobile units to and from a telecommunications network via a mobile switching center. The coverage area of a typical cellular telecommunications system is divided into several cells. Different communications resources such as frequencies are often allocated to each cell to maximize communications system resources. When a mobile unit moves from a first cell to a second cell, a handoff is performed to assign new system resources associated with the second cell.
A handoff involves the execution of a set of negotiation instructions between the mobile unit and one or more governing base stations and/or mobile switching centers. Cellular telecommunications systems generally require timely handoff procedures to maximize the utilization of system resources. Efficient and timely handoff procedures are becoming increasingly important as smaller cells are deployed to meet demands for increased communications system capacity. Use of the smaller cells increases the number of cell boundary crossings and frequency assignments thereby increasing the need for efficient and cost-effective handoff triggering mechanisms.
Timely handoff mechanisms are particularly important for systems employing `hard handoff` procedures. Hard handoff procedures are used to transfer an ongoing call between adjacent cells, different frequency assignments and/or different frame offsets, or to direct a mobile station from a code division multiple access (CDMA) forward traffic channel to an analog voice channel. In a hard handoff, the first link with the first cell is broken and then second link is established. (In a `soft handoff`, the first link is maintained until the second link is established and there is a time during which the first link and second link are maintained simultaneously.) In the case of hard handoff a large delay between the dropping of the first link and the establishment of the second link may result in unacceptable communications system service quality.
To enhance telecommunications system capacity and service quality, multi-layer systems with macrocells overlaying microcells are often employed. Clever uses of the layers can lead to increased end-user performance and system capacity. For example, stationary users can be assigned to microcells so that they operate at reduced power and cause significantly less interference. When the microcellular capacity is exhausted, overflow traffic is assigned to the macrocells. Typically, the microcells will have more frequency assignments than the macrocells. Hard handoff is implemented when a mobile unit crosses between the two layers in the multi-layer system.
To facilitate hard handoff between adjacent cells, a handoff beacon is often employed. A beacon in each cell broadcasts a signal having a limited range about the cell. When a mobile unit in a first cell detects a beacon from a second cell, the telephone is handed off to the second cell. Unfortunately, the beacon mechanism makes inefficient use of system resources. The frequency band allocated for such beacons does not currently support traffic such as voice calls. In addition, the beacons may require additional expensive broadcasting hardware. These limitations often limit the coverage and capacity of telecommunications system cells.
Alternatively, a transition cell may be implemented between two cells. The transition cell overlaps the two cells and maintains a special pilot offset signal detectable by a mobile unit. When the mobile unit detects the pilot offset, hard handoff is triggered. The transition cell must substantially overlap the first and second cells. The overlap represents inefficient and redundant use of system resources. In addition, the transition cell doesn't support system traffic.
Hence, a need exists in the art for a fast, efficient and cost-effective system for triggering hard handoff in a cellular telecommunications system.